Cationic Alkylaluminum-Complexed Zirconocene Hydrides: NMR-Spectroscopic Identification, Crystallographic Structure Determination, and Interconversion with Other Zirconocene Cations

The ansa-zirconocene complex rac-Me2Si(1-indenyl)2ZrCl2 ((SBI)ZrCl2) reacts with diisobutylaluminum hydride and trityl tetrakis(perfluorophenyl)borate in hydrocarbon solutions to give the cation [(SBI)Zr(μ-H)3(AliBu2)2]+, the identity of which is derived from NMR data and supported by a crystallographic structure determination. Analogous reactions proceed with many other zirconocene dichloride complexes. [(SBI)Zr(μ-H)3(AliBu2)2]+ reacts reversibly with ClAliBu2 to give the dichloro-bridged cation [(SBI)Zr(μ-Cl)2AliBu2]+. Reaction with AlMe3 first leads to mixed-alkyl species [(SBI)Zr(μ-H)3(AlMexiBu2−x)2]+ by exchange of alkyl groups between aluminum centers. At higher AlMe3/Zr ratios, [(SBI)Zr(μ-Me)2AlMe2]+, a constituent of methylalumoxane-activated catalyst systems, is formed in an equilibrium, in which the hydride cation [(SBI)Zr(μ-H)3(AlR2)2]+ strongly predominates at comparable HAliBu2 and AlMe3 concentrations, thus implicating the presence of this hydride cation in olefin polymerization catalyst systems.